This invention relates to fuel cells in general, and more particularly, to a system and method for regulating the hydration level of the polymer electrolyte membrane during operation.
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. The reaction produces only water as a by-product, which has an environmental advantage, and has attracted fuel cells to an enormous potential market for portable energy. A typical fuel cell consists of a fuel electrode (anode) and an oxidant electrode (cathode), separated by an ion-conducting electrolyte. The electrodes are connected electrically to a load (such as an electronic circuit) by external conductors. In these circuit conductors, electric current is transported by the flow of electrons, whereas in the electrolyte it is transported by the flow of ions, such as the hydrogen ion (H+) in acid electrolytes, or the hydroxyl ion (OHxe2x88x92) in alkaline electrolytes. Gaseous hydrogen has become the fuel of choice for most applications, because of its high reactivity in the presence of suitable catalysts and because of its high energy density, and the most common oxidant is gaseous oxygen, which is readily and economically available from the air for fuel cells used in terrestrial applications. The ionic conductivity of the electrolyte is a critical parameter that determines the efficiency and operating condition of a fuel cell. In the case of solid polymer electrolyte membrane (PEM) fuel cells, the combined assembly comprising the PEM, the cathode, and the anode is known as the membrane electrode assembly (MEA).
One major problem associated with a robust fuel cell design is management of water during the operation. In PEM fuel cells, the ionic conductivity of the electrolyte membrane is dependent on the hydration level of the membrane as water molecules are involved in the transport of hydrogen ions across the electrolyte. Typically, fuel cells operate best when fully hydrated and at ambient temperatures, but this can be a tenuous balance. If the by-product water is not removed from the MEA fast enough, the MEA xe2x80x9cfloodsxe2x80x9d (too much water generated during fuel cell operation) and the performance of the fuel cell decreases and/or the fuel cell ceases to function. At the other extreme (xe2x80x9cdryingxe2x80x9dxe2x80x94not enough water generated during fuel cell operation), if the PEM is not hydrated enough, the ionic conductivity of the PEM is poor and the transfer rate of ions across the membrane is slow or non-existent, again resulting in poor performance. The problems with maintaining the optimum hydration of the PEM are well known, and many have attempted to solve these problems by various mechanical schemes and by elaborate electronic controls. This is but one of the hurdles that have prevented the widespread adoption fuel cells in the modern world, despite their promise of pollution-free and renewable electricity. A means to control the hydration of the PEM is needed, and may help push the development of a robust fuel cell into product realization.